This invention relates generally to the field of x-ray diffraction analysis and, more particularly, to the use of reflection mode diffraction for combinatorial screening.
Combinatorial chemistry refers generally to the testing of a large number of related materials and the storing of analytical data resulting from the tests. It is desirable in a number of analytical disciplines to perform rapid screening tests on tens, hundreds, or even thousands of related samples to evaluate variations of composition, structure and property. The results are then used to build a material library. X-ray diffraction is a useful technique for performing sample analysis because extensive information may be acquired from the diffraction pattern, and the testing is fast and non-destructive. However, many combinatorial chemistry applications, such as the analysis of certain pharmaceutical materials, require x-ray diffraction screening in a low Bragg angle range.
In low angle diffraction measurement, the incident x-ray beam is spread out over the sample surface in an area much larger than the cross-sectional size of the original x-ray beam. This becomes a problem in combinatorial chemistry applications because it is desirable to use a two-dimensional sample tray having many sample cells that lie adjacent to each other. Because the sample cells are close to each other, the spread beam may cause cross contamination in the collected diffraction data. That is, diffracted x-ray energy from a sample not under test could accidentally be detected by the detector, thereby affecting the results of the analysis of the sample under test. Conventionally, a shield (sometimes referred to as a xe2x80x9cknife-edgexe2x80x9d) has been used to block, from reaching the detector, diffracted x-ray energy from all but the directions associated with the sample under test. This shield helps to improve the signal-to-noise ratio of the detected signal.
Although the shield is useful in blocking stray x-ray energy from samples surrounding a sample under test, it also acts as an obstruction to a view of the sample tray. In the present invention, a retractable shield is used that may be located in proper position to block stray x-ray energy during a test, but is then retracted while the sample tray is repositioned. This retraction of the shield allows a clear view of the sample tray. This, in turn, facilitates the use of a laser and video camera system used for automated alignment of the sample tray.
In accordance with the invention, an x-ray diffraction analysis apparatus performs an analysis on a plurality of samples in a multiple-cell sample holder. The apparatus includes a sample support upon which the sample holder may be located. Preferably, the sample support is automatically controlled to allow positioning of the sample for either fine-tuning for the sample testing itself, or for moving a new sample into testing position. When properly positioned, the sample under test undergoes x-ray diffraction analysis. An x-ray source is provided that directs x-ray radiation toward the sample under test, and x-ray radiation diffracted from the sample is detected by an x-ray detector.
In order to prevent x-ray radiation other than that diffracted from the a sample under test from reaching the detector, the retractable x-ray shield is placed in an extended position adjacent to the sample. The x-ray shield is movable between the extended position and a retracted position in which it is outside the field of view of an observation apparatus. Typically, such apparatus is positioned directly above the sample holder although, in general, the unobstructed view is from a position substantially along an axis perpendicular to the plane of the sample holder that intersects the center of the sample. If the sample holder is held in a horizontal plane, this means an unobstructed view of the sample from directly above it. The observation system may be a video camera/microscope combination used with a laser light source. Such an observation system can be used to precisely position a desired one of the samples relative to the testing apparatus.
Some of the desirable features of the shield include a tapering toward the sample so that a xe2x80x9cknife-edgexe2x80x9d is the closest part of the shield to the sample. The positioning of the shield in the extended position is such that it blocks diffracted x-ray energy from samples closer to the x-ray source than the sample under test, and it blocks x-ray energy from the x-ray source that is directed toward a sample further from the x-ray source than the first sample. A particularly suitable gap distance between the surface of the sample and the knife-edge may be determined from other parameters of the system. Such a distance may be equal to the height of a triangle having a base of length S, where S is the distance between the centers of samples adjacent to each other along the primary dimension separating the x-ray source and the detector. The two sides of the triangle have respective angles (relative to the base) of xcex81 and xcex82, where xcex81 is the angle between the primary emitting direction of the x-ray source and the surface of the sample holder, and xcex82 is the angle between the primary detection direction of the detector and the surface of the sample holder. The height of the triangle is the distance from the base to the point at which these two sides intersect.
Like the sample support, the shield also preferably includes an actuator by which it may be automatically moved. This allows the system to perform automatic testing of some or all of the samples of the sample holder. The sample holder would be positioned on the sample support with a first sample roughly aligned at a testing position. The positioning of the first sample would then be examined using the observation apparatus. Using the laser to illuminate the sample surface, the output from the video camera would provide feedback for a fine-tuning of the sample position by movement of the sample support with the sample position adjuster. This positioning is done with the shield in the retracted position. Once the positioning step is complete, the actuator for the shield is activated to move it into the extended position. The x-ray source and detector are then activated, and the diffracted x-rays from the sample under test are detected and the diffraction data recorded. The actuator then moves the shield to the retracted position, and the next sample to be tested is moved into the testing position. The steps of positioning and testing are then repeated for the new sample and for each sample after that until the final sample is reached. Once the system determines that the final sample has been reached, testing routine halts.